I have two boys and I don't think they smell strongly, of course their poop and pee does a bit but compared to other animals I find it quite harmless. My boys live inside, but in the summer we set them up with a tube so they can go between the indoors and their run on the grass during the day. They definitely love doing this and will popcorn excitedly, but I don't think it's necessary. I think I know the Youtuber you're talking about, and all her guinea pigs are so very happy despite not having access to a garden, so clearly it can work. One thing I would say though is they do love having large spaces to run around, so it would be great if you have a safe area indoors where you can let them out of their cage for a while each day, or a few times a week. Of course, they will pee and poop on your floor if you do this so it requires some cleanup after.
In terms of time, they do need regular socialisation with humans, but they don't need somebody to be around all day (I don't know how this would be with a pig with any special needs) as long as they aren't kept as a single guinea pig (which shouldn't happen for any significant length of time unless they can't be bonded or some other rare circumstance). I think even if you're going into the office 5 days a week, normal healthy guinea pigs who are settled into their home will be perfectly fine with interaction and feeding just in the mornings and evenings around work during the weekdays (and hopefully some extra interaction at weekends). It is good to get into a routine of feeding them at consistent times to help them feel secure.
How often you need to clean out the cage will depend on it's size, as if it's a huge cage there's more room for poop, but expect to need to do it roughly every other day, as they do poop and pee a lot. How long this will take really depends on your setup. Our boys have fleece on the bottom of their cage which can be picked up, shaken off into a black sack and then washed in a wash bag in the washing machine (to keep all the fluff and hay out of your machine!). So this is quite fast to do, and once you've put in a fresh clean fleece it's just about arranging all of their "furniture" back into their cage and setting up their hay and water. I'd say it takes around half an hour each time. They need to be fed twice a day, with specific guinea pig pellets and greens such as romaine lettuce, kale, parsley etc. They also need a constant supply of fresh water (from one of those rodent bottles, freshened at least once a day) and a constant supply of hay to munch on, preferably timothy hay as it's a high quality grass hay. They also need their nails clipped regularly as they grow constantly - how often you need to do this will depend on how quickly they grow and how long they are to start with. As a general rule, your piggies nails should be short enough that they don't clack against the floor when they walk. It can be quite stressful, as if you cut them too short they will bleed. It took me a while to feel confident doing it, but I can do them both in about 10 minutes now, with my partner holding them for me and treats for bribery. Depending on the type of pig you have, you may also need to trim their fur. I have an Abyssinian and his fur gets quite long and will trail in the mess, so I trim his fur a little each time I cut his nails to keep it nice. I've heard conflicting information about bathing guinea pigs, but general guidance seems to be to bath them around twice a year if they need it. It will probably depend on the individual piggy if/when they need it - again, a long haired pig may require it more often. A lot of GPs find the nail clipping and bathing in particular to be stressful so you need to be able to stay calm and hold them safely, and it helps to give them treats like carrot or red peppers to sweeten the deal a little. Oh, they also need plenty of safe wooden items in their cage to chew on, so they can keep their teeth short (their teeth grow constantly). Mine can chew through a wicker tunnel in a few days!
Wow, that was long! I hope that all makes sense. I want to add though that I love my piggies so, so much. They're like my babies and even though they do require a lot of work, it feels so so worth it when they come up to me and put their little paws on me asking for a treat! Or when they wheek and popcorn because they've got an interesting new cage layout, or a fun new toy to crunch on. Maybe I'm just a melt but seeing them super happy makes me cry sometimes haha! Whatever you decide good luck, fostering is a lovely thing to do xx